GENETICS AND HEREDITY Heredity and Genetics Heredity is

GENETICS AND HEREDITY

Heredity and Genetics • Heredity is the passing of physical characteristics from parents to offspring. • Genetics is the scientific study of heredity.

Mendel • Gregor Mendel, an Austrian monk of the nineteenth century, made the discoveries that is the foundation of our knowledge of genetics.

Mendel’s Experiments • He did his experiments because he wondered why pea plants had different characteristics. – Tall and short plants – Green and yellow seeds – Round (smooth) and wrinkled seeds • Each different form of a characteristic is called a trait.

Mendel’s Experiments • Fertilization is the process where an egg cell and a sperm cell join together. • Pollination is the process of the pollen reaching the pistil of a flower. • Pea plants are usually self-pollinating, meaning the pollen of a flower lands on the pistil of the same flower. • Mendel developed a method of crosspollination. – He removed pollen from the flower of one plant and then brushed the pollen onto a flower on a second plant.

Crossing Pea Plants • Mendel decided to cross plants with opposite traits, for example tall and short plants. • He began his experiments with purebred plants. • Purebred organisms are the offspring of many generations that have the same trait. – Example: Purebred short plants always come from short parent plants. – Purebred individuals are also called “true breeding” individuals.

The F 1 Offspring • In Mendel’s experiments, the purebred parent plants are called the parental generation or P generation. – Example: Mendel crossed a purebred tall plant with a purebred short plant. • The offspring of the P generation are called the first filial (Latin for daughter or son), or F 1 generation. – Example: In Mendel’s F 1 generation, all of the plants were tall. – Even though one of the parents was short, that trait seemed to disappear in the F 1 generation.

The F 2 Generation • Mendel let the fully grown F 1 plants to selfpollinate. • The second filial, or F 2 generation were a mix of tall and short plants. • The short trait reappeared even though none of the parents were short. • After counting the F 2 plants, Mendel noted that ¾ of the plants were tall and ¼ of the plants were short.

Experiments with Other Traits • Mendel did hundreds of crosses looking at other traits. • In all of his crosses, only one form of the trait appeared in the F 1 generation, but that trait reappeared in the F 2 generation in about ¼ of the plants.

Dominant and Recessive Alleles • Because of his experiments, Mendel concluded that individual “factors” must control the inheritance of traits. • He also reasoned that the factors that control each trait exists in pairs, one factor from each parent. • Based on the results of his experiments, Mendel concluded that one factor in each pair can mask, or hide, the other factor. – Example: The tallness factor masked the shortness factor.

Genes and Alleles • Today, scientists call the factors that control a trait a gene. • The two different forms of a gene are called alleles. • Each pea plant inherits one allele from each parent. – A pea plant could inherit 2 tall alleles, 2 short alleles, or 1 of each.

Genes and Alleles • An organism’s traits are controlled by the alleles it inherits from its parents. • Some alleles are dominant. – Dominant alleles are those whose trait always shows up in the organism when that allele is present. • Other alleles are recessive. – Recessive alleles are those whose traits are hidden whenever the dominant allele is present. • Recessive traits only show up if the organism does not have the dominant allele. In other words, the organism has two recessive alleles.

Genes and Alleles • In Mendel’s crosses, the allele for tall plants is dominant over the allele for short plants. – Only plants that inherit two short alleles will be short. Plants that receive one or two dominant alleles will be tall.

Alleles in Mendel’s Crosses • In Mendel’s experiments, the purebred tall plants had 2 alleles for being tall, while the purebred short plants had 2 alleles for being short. • All of the plants from the F 1 generation received one tall allele and one short allele. • Organisms that has two different alleles for a trait is called hybrid. – All of the hybrid plants were tall because they received 1 tall and 1 short allele, but the tall is dominant over the short.

Alleles in Mendel’s Crosses • When the F 1 plants self-pollinated, some of the F 2 plants received two dominant alleles for tallness. – These plants were tall. • Other F 2 plants received one dominant and one recessive allele. – These plants were tall. • The rest of the F 2 plants received two alleles for shortness. – These plants were short.

Symbols for Alleles • Letters are used to represent alleles. • Dominant alleles are represented by capital letters. – The tall allele would be T. • Recessive alleles are represented by lowercase letters. – The short allele would be t. • The alleles an organism receives for a trait are represented by a combination of letters. – The combination of alleles possible for pea plants are TT, Tt, and tt.

Homozygous and Heterozygous • An organism is said to be homozygous for a trait if both alleles are identical. – Example: TT and tt are homozygous allele combinations. • TT is homozygous dominant. • tt is homozygous recessive. • An organisms is said to be heterozygous for a trait if the organism has both a dominant and recessive allele. – Example: Tt is a heterozygous allele combination. – All hybrids are heterozygous individuals.

Why Mendel was Important? • Before Mendel, scientists thought that the traits of an individual were simply a blend of the parent’s traits. – Example: If a tall plant and a short plant reproduced, they would make medium sized plants. • Because of Mendel’s experiments, traits are determined by individual, separate alleles inherited from each parent. • Mendel’s discovery was not recognized during his lifetime. – His work was rediscovered in 1900. – Mendel is known as the Father of Genetics.

Probability and Genetics • Mendel carefully counted all of the offspring from every cross he carried out. • When he crossed two tall hybrid plants, ¾ of the F 2 generation were tall and ¼ were short. • Each time he repeated the cross, he obtained similar results. • He realized that probability applied to his work.

Probability and Genetics • Mendel could say that the probability of producing a tall plant in the F 2 generation was 3 in 4. • The probability of producing a short plant in the F 2 generation was 1 in 4. • Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses.

Punnett Squares • A Punnett Square is a chart that shows all the combinations of alleles that can result from a genetic cross. • Geneticists use these to show all the possible outcomes of a genetic cross, and to determine the probability of a particular outcome.

How to Make a Punnett Square • Draw a square and divide it into 4 smaller squares.

How to Make a Punnett Square • Place the alleles from one parent along the top of the Punnett square. – Make sure that only one letter is above each box. T t • Place the alleles from the other parent along the left side of the square. – Make sure that only one letter is beside each box. t T

How to Make a Punnett Square • Copy the alleles from the top into each box under them. T t t T T T

How to Make a Punnett Square • Now place each letter on the left of the box into the boxes to the right of them. • When you are finished, you should have two letters in each box. • You always should write the dominant allele on the left-hand side. – Tt instead of t. T. T t t Tt Tt Tt

How to Make a Punnett Square • The boxes in the Punnett square represent all the possible combinations of alleles that the offspring can inherit. • In this Punnett square, we see the results of crossing a purebred tall plant with a purebred short plant. – All of the offspring are hybrid tall plants. – From this cross, 4 in 4, or 100% will be tall. T t t Tt Tt Tt

Using a Punnett Square • • • In a genetic cross, the allele that each parent will pass on to its offspring is based on probability. In the Punnett square to the right, there is a 3 in 4 chance, or 75% chance that the offspring would inherit the tall trait. The Punnett square represents the chances each time a pair reproduces. – This does not mean that if the pair to the right had 4 offspring, 3 would be tall and 1 would be short. – It says that each time they reproduce there is a 75% chance for tall plants and 25% chance for short. T T t TT Tt tt

Phenotypes and Genotypes • A pheontype is an organism’s physical appearance or visible traits. – Example: tall, short, purple flowers, white flowers, wrinkled seeds, round seeds, black fur, white fur • A genotype is its genetic makeup or allele combinations. In other words, the combination of letters. – Example: TT, Tt, tt, BB, Bb, bb, RR, Rr, rr, WW, Ww, ww

Codominance • For all the traits that Mendel studied, one allele was dominant while the other was recessive. • This does not happen 100% of the time. • In codominance, the alleles are not dominant nor recessive. – As a result, both alleles are expressed in the offspring.

Genetic Laws The Law of Dominance states that when an organism has two different alleles for a trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant. The allele whose expression is overshadowed is said to be recessive.

Genetics Laws The Law of Segregation states that the alleles for a trait separate when gametes (egg and sperm) are formed. These allele pairs are then randomly united at fertilization. Mendel arrived at this conclusion by performing monohybrid crosses. These cross-pollination experiments were with pea plants that differed in one trait, such as pod color.

Genetics Laws The Law of Independent Assortment states that alleles for different traits are distributed to sex cells and offspring independently of one another. – This means that the inheritance of one trait has nothing to do with the inheritance of another. • Example: Just because a pea plant inherits the tall trait does not mean that they must also inherit the trait for having wrinkled seeds.

Genetic Disorders and Recessive Genes • Many genetic disorders are caused by recessive genes. • If an offspring receives two recessive alleles from the parents, the child inherits the disease. • If a person is heterozygous, he/she will not show the symptoms. – These people are known as carriers.

Cystic Fibrosis • This is a genetic disorder in which the body produces abnormally thick mucus in the lungs and intestines. – The mucus fills the lungs and makes it hard to breathe. • It is caused by a recessive allele on one chromosome. – It is the result of a mutation in which three bases are removed from DNA.

Sickle Cell Disease • Sickle cell anemia results from a substitution mutation of the DNA in the sex cells. This has resulted in a recessive trait. • Sickle cell commonly affects people of African, Indian, and Mediterranean descent. • It causes the red blood cells to become sickleshaped. – This prevents the blood from passing normally through the capillaries, resulting in oxygen not being passed on to the tissues.

Hemophilia • This is a genetic disorder in which a person’s blood clots very slowly or not at all. • They do not produce one of the proteins needed for normal blood clotting. – Have a high risk of internal bleeding from small bumps and bruises. • Caused by a recessive allele on the X chromosome, making it a sex-linked disorder. – Occurs more often in males than females.

Heredity and Meiosis • Sometimes mistakes happen during meiosis, the production of egg and sperm cells. • This can result in individuals having more or fewer chromosomes than normal. – Individuals with Down’s Syndrome have an extra copy of chromosome 21. • This results in a variety of physical and/or mental conditions.

Sex Chromosomes • The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits. • The sex chromosomes are the only pair that do not always match. – In females, the chromosomes match. The female genotype is XX. – In males, the chromosomes do not match. The male genotype is XY.

Sex-Linked Genes • Genes on the X and Y chromosomes are called sex-linked genes because their alleles are passed from parent to child on a sex chromosome. • Traits controlled by sex-linked genes are called sex-linked traits. • One sex-linked trait is red-green colorblindness. – A person with this trait cannot distinguish between the colors red and green.

Pedigrees • A pedigree is a chart or “family tree” that tracks which members of a family have a particular trait. • Pedigrees include two or more generations. • Females are represented by circles, while males are represented by squares. • Those with a trait are shaded, while those that do not have a trait are left clear. • If the organism is a carrier of a trait, but does not show the trait, their symbol is only shaded halfway.

A Pedigree for Albinism (A condition where the skin, hair, and eyes lack normal coloring)